What does it mean when a scanner has a "trunking" feature?

Question

I am looking to buy a radio frequency scanner and I've noticed some have trunking. What is trunking?

Answer 1

Trunking is a radio system that lets lots of different users share a small number of frequencies.  When someone in group A sends a message all of the radios that belong to group A are sent to an unused frequency by a system controller.  If someone in group B also sends a message they're sent to a different open frequency.  Each time a group transmits they may wind up on a different frequency but since all the radios go to that frequency at the same time it doesn't matter.  They all hear the entire conversation.  This way 20 or so different groups of people can share say 5 different channels without hearing or being bothered by each other.  A trunking scanner can decode the system command for a group to go to a certain frequency and follow along so you hear the entire conversation as well.  Otherwise you might hear one transmission from one group and another from another group and so on.  Nothing would make sense because you wouldn't hear the complete conversation.  There are many trunking systems like this, especially in larger cities.

Answer 2

That is a question that requires a long answer, but I'll make it as brief as possible. Trunked radios work on a different system than regular fixed frequency radios. The radio use remote, fixed  sites that contain channel units. Each channel unit has a separate transmitter and receiver, each working on a different frequency. When a call is made, the radio is told to tune in the transmit frequency of whichever channel is assigned to broadcast the call. It then transmits on the frequency that is assigned for receiving. This is usually done on multiple sites, so each site is sending and receiving the conversation on different frequencies. There is normally a central node that routes the audio to keep it from getting mixed up. That means that the radios have to be capable of working on a multiple frequencies. A scanner that can listen to trunked radios is simply one that can automatically or be programmed to listen to one conversation spread out over more than one frequency.

This leaves out quite a bit, but a full explanation would take days.